Pressure vessel



Oct. 29, 1935. J SHALER 2,018,976

PRESSURE VESSEL Filed Sept. 6, 1933 2 Sheets-Sheet l i FL? 2 I Juh A M 4w y- 1 lhflenior iQ Zlllflqliii Oct. 29, 1935. J. T. SHALER 2,018,976

PRESSURE VESSEL Filed Sept. 6, 1935 2 Sheetg-Sheet 2 Aiiorzz ey Patented Oct. 29, 1935 UNITED STATES PATENT OFFICE.

PRESSURE VESSEL John T. Shaler, Berkeley,

Calif., assignor to Standard Oil Company of California, San Francisco, Calif a corporation of Delaware This invention relates to pressure Vessels such as are used for high temperature and pressure service as in the treating of petroleum to produce lower boiling point products, and particularly to a diaphragm construction whereby the temperature and liability to corrosion of certain parts of such a vessel are materially reduced.

In the performance of the various chemical processes involving the application of high pressures and temperatures, such as in the cracking of hydrocarbons, it becomes necessary to provide pressure vessels that will successfully and economically withstand the ever-increasing pressures and temperatures for long periods. This requirement is complicated by at least two important factors; first, the materials undergoing treatment may be extremely corrosive to the material of the vessel, under the operating conditions; and second, the material of the vessel may be of such nature as to lose an appreciable part of its normal strength at moderate temperatures when it is exposed to the high temperatures of the process.

The first diificulty, corrosion, may be greatly reduced by lining the vessel with a. material, usually a metal or alloy, that resists the particular type of attack involved. This may be done, in the case of carbon steel vessels used in cracking processes, by electrolytic deposition of a metal such as chromium on the interior of the shell. A more recent method is the lining of the vessel with sheets of a chromium iron alloy, spot welding them at frequent intervals and lap or butt welding all seams to form a tight internal sheath or skin.

There are two general methods of maintaining a safe strength of a steel pressure vessel at the high temperatures (800 F. to 900 F. and above) that are required for certain processes. The first is by the use of low working stresses; in other words, making the vessel excessively thick and therefore strong at ordinary temperatures, so that the remaining strength at the operating conditions will be ample for safety. The second method is the use of special alloy steels, such as the chromium-nickel-iron group, which, with proper fabrication and heat treatment, will have a higher tensile strength at temperatures up to 1000 F. to 1200 F. than carbon steel at much lower temperatures. Both of these methodsare expensive and troublesome.

A further complication, in the design of pressure vessels, is that, for the same metal thickness, the cylindrical shell is usually stronger than the semi-elliptical or bumped heads, particularly because of the number of openings, nozzles, manholes, etc., that are necessary in the heads. The latest practice is the use of so-called basket handle shapes, or modified ellipsoids of revolution. These offer the correct mathematical design, but, because of their shape, are very difficult and expensive to protect against corrosion, by either the electroplating or sheathing methods outlined above. The cells for electroplating require accurate spacing and conformity to the metal surfaces to be coated. This requires a large investment in special equipment and time. When sheets of fiat metal are used for sheating the variously curved surfaces they must be cut into very small sections and require careful and painstakl5 ing Work to make a successful job. Special cements are sometimes applied by guniting, but this material spalls and cracks under the conditions of the process.

It has been discovered, however, that the gases and vapors which cause the severe corrosion encountered in the upper sections and heads of vertical cylindrical vessels, particularly those used in hydrocarbon cracking processes, will quickly become exhausted of their corrosive properties if they are kept in quiet contact with the parts concerned, and are not allowed to circulate and mix with the turbulent products in the body of the vessel. To this end, therefore, it is an object of this invention to provide means, such as a diaphragm or baflle, separating the space beneath the head of the vessel from the cylindrical body, but allowing passages therethrough sufiicient for drainage and the equalization of pressure differences between the two spaces. This has shown a remarkable reduction in the corrosion of the material of the head, and in many cases obviates the necessity for any other special lining for that area.

A further result and object of this invention is the property of such a partition to reduce the temperature of the metal in the head of the vessel to a point several hundred degrees below that of the cylindrical shell or the contents of the vessel. Actual tests over a period of 60 days on an electrically welded carbon steel evaporator drum in a cracking process, 10 ft. diameter and 30 ft. long, the cylindrical portion 2 inches thick and the top head 2 inches thick fitted with such balile showed that-- l. The vapor temperature below the baffle was 880 3?. maximum, 845F. average.

2'. The vapor temperature above the baffle was 656 F. maximum, 540 F. average.

3. The skin temperature of the metal head,

no outer insulation, no baffle, was 765 mum, 740 F. average.

4. The skin temperatureof the metal head, no outer insulation, with baflie, was 540 F. maximum, 510 F. average.

These test results, and operation over a period of several thousand hours indicating greatly decreased corrosion of the head, show the benefit of the baffles in such apparatus.

Furthermore, it is well known that carbon steel may be actually stronger at temperatures of approximately 500 F. than at atmospheric temperatures while above 700 F. the strength drops off rapidly with increasing temperatures. For example, a 0.13% carbon steel (Curve A. Fig. 1, Trans. A.'S. M. E. FSP-52-35 p. 295) at 100 F. showed a tensile strength of 54,000 lb. per sq. inch; at 500 F.--62,000 lbs. per sq. inch; and at 765 F.-- 0,000 lbs. per sq. inch. This last was a reduction in tensile strength of 22,000 lbs. per sq. inch or 35% from that at 500 F.

' Another property of metals, that of creep or continued deformation at stresses below the elastic limit, is profoundly afiected by higher temperatures than those mentioned above. For example, the creep Value or stress to cause a 1% elongation in 100,000 hours for a certain 24% carbon steel (Curve 16, Fig. 6. Trans. A. S. M. E. FSP-52-35 p. 298) was 32,000 lbs. per sq. inch at 765 F. and dropped to 16,000 lbs. per sq. inch at 900 F., a reduction of 16,000 lbs. per sq. inch or At temperatures above about 900 F., the creep rate is considered the important factor in safe design. Below that figure the ultimate tensile stress, mentioned in the first example above, is believed to govern.

It is thus seen that the two major objects of the invention, reduction in corrosion and reduction in temperature of the head of the vessel are of great importance in both the design and the safe operation of such equipment.

One embodiment of this invention is shown in the attached drawings, where Fig. 1 is a partial vertical section through a vertical cylindrical steel pressure vessel, such as is used in hydrocarbon cracking processes.

Fig. 2 shows a horizontal section through the vessel on line IIII of Figure 1 and above the diaphragm.

Fig. 3 is a detail of the manhole and cover provided in the diaphragm for access to the space below it.

Fig. 4 shows a detail of the fiexible sleeves used to carry liquids, vapors and gases through the space protected by the diaphragm to the outlet and inlet nozzles in the head of the drum.

Fig. 1 shows the cylindrical portion of the drum at I and the semi-elliptical head at 2, the latter being provided with an access manhole 3 and one or more pipe connections or nozzles 4. The corrosion resistant sheet lining 5, in this case an 11-13% chromium iron alloy, for the cylindrical section of the drum, may be ended at the point Where the baffle plates 0 are attached to the shell, or it may be extended into the head 2 and manhole 3 (not shown).

The baffle or diaphragm 6 may be composed of flanged segments of 11-13% chromium iron alloy sheet, properly heat treated according to well known practice. The segments are electrically welded at their outer ends I to the chromium bolts or welds.

iron alloy linings of the drum with suitable alloy rod and are fastened to each other by rivets, A flanged stiffening ring 8 of the same material is provided at the inner ends of the F. maxisegments to form a reinforcement and to act as a manhole through which access may be had to the space below the diaphragm. The latter space may be empty, or provided with packing, plates and rings, trays, bubble decks, or whatever filling is required for the process. The diaphragm is sloped from its periphery to the manhole in the center, to provide drainage for the condensate which may accumulate on its upper surface. Brackets 9 are provided to support and stifien each segment.

The inner ends of several of the segments are turned upward to form centering lugs In for the manhole cover Ii, also of chromium iron alloy, which is placed over the hole when the equipment is in operation. The lower rim of the cover is serrated or notched as shown at l2 (Figure 3) to allow passage of condensate, and also afford space for equalization of gas or vapor pressures above and below the diaphragm. A strap handle I3 is provided for convenience in handling the manhole cover H through the manway 3 in the outer drum. It may also be advisable to load or weight the cover H by means of a heavy plate I4, of scrap carbon steel, to insure its remaining seated during parts of a process when turbulence or accidental disturbance below the diaphragm would tend to lift the cover.

Communication between the pipe connections or nozzles 4 in the head of the drum, and the space below the diaphragm 6 is made by relatively flexible sliding sleeve assemblies as illustrated in Fig. 4. In the example shown one of these consists of a chromium iron alloy tube l5 which fits tightly in the bore of the nozzle 4 and extends downward into the space above. the diaphragm. Surrounding the tube l5 fora portion of itslength is a second tube or sleeve l6 which extends downward through a hole in the diaphragm 6 into the space below it. This. second sleeve I6 is held from dropping downward by an alloy-clamp ll surrounding it and resting upon the diaphragm 6. A chromium iron alloy bolt i8 is fastened through both clamp I1 and sleeve I6 to act as a further support.

It has been found desirable to make all the joints and seams in the diaphragm structure as tight as possible in order to limit all communication between the spaces above and below the partition to the serrations around the central manhole cover ID. This has been found to reduce circulation of the gases and vapors and greatly increase the effectiveness of the diaphragm, both to reduce corrosion and to lower the temperature of the top head.

The essence of the invention appears to lie in the provision in cylindrical vessels operating under high pressure, temperature, and corrosive conditions, of a diaphragm of corrosion resistant material to produce a dead gas insulating space above the cylindrical portion of said vessels with means to permit equalization of pressure above and below said diaphragm, whereby the temperature above the diaphragm is reduced and corrosion of that portion of the vessel is retarded.

Although a specific construction embodying this invention has been described and illustrated, it is to be understood that the invention is not limited to that arrangement, and all such modifications and changes as come within the scope of the following claims are embraced thereby.

I claim: a

1. An oil refining vessel comprising a cylindrical shell, an outwardly curved head on said shell,

a diaphragm across said shell, said diaphragm 75 cooperating with said head to define a quiescent vapor space, an opening in said diaphragm, a removable cover for said opening, ports in said cover communicating between said quiescent vapor space and the space within said shell, and a conduit to convey vapor from the space within said shell out of contact with said quiescent vapor.

2. An oil refining vessel comprising a cylindrical shell, an outwardly curved head on said shell, a diaphragm across said shell, said diaphragm being sloped from its outer perimeter toward its center, and cooperating with said head to define a quiescent vapor space, a manhole in the center of said diaphragm, a cover for said manhole, ports in said cover, and a substantially vapor tight conduit extending through said diaphragm and said head to carry vapor from the space within said shell.

3. An oil refining vessel comprising a cylindrical shell, an outwardly curved head on said shell, a diaphragm across said shell and cooperating with said head to define a quiescent vapor space, means in said diaphragm to provide a restricted communication between said quiescent vapor space and the interior of said shell, and an outlet to carry vapor from said shell out of contact with the vapor between said head an said diaphragm.

4. An oil refining vessel according to claim 3, in which said last named outlet comprises a conduit passing through said diaphragm and said head.

5. An oil refining vessel comprising a cylindrical shell, 2. head on said shell, a diaphragm across said shell and spaced from said head to define a quiescent vapor space therebetween, means in said diaphragm to provide a restricted communication between said quiescent vapor space and the interior of said shell, and an outlet to carry vapor from said shell out of contact with the vapor in said quiescent vapor space. 6. An oil refining vessel according to claim 5 in which said last named outlet comprises a con- .duit passing'through sai-d diaphragm and said head.

JOHN T. SHALER. 

